1. Field of the Invention
The present invention relates to a television signal converting apparatus for converting the television signal of a certain system into that of another system.
2. Related Background Art
For a conversion of the television signal of a high definition television system, such as of so-called high-vision system (hereinafter called HDTV signal) into the television signal with a fewer number of scanning lines, such as of the NTSC system (hereinafter called NTSCTV signal), there has already been proposed a television signal converting apparatus of the structure shown in FIG. 1.
FIG. 1 is a schematic block diagram of a conventional television signal converting apparatus, in which the luminance signal Y and the color difference signals P.sub.R, P.sub.B of the HDTV signal are entered through terminals 100, 101, 102 and converted in a matrix circuit 103 into R, G and B signals. Said R, G and B signals obtained from the matrix circuit 103 are respectively converted into digital signals by A/D converters 104, 105, 106, then subjected to a conversion in the aspect ratio from 16:9 based on the HDTV signal to 4:3 based on the NTSCTV signal in aspect ratio conversion circuit 107, 108, 109, then to a conversion of number of scanning lines from 1,125 lines based on the HDTV signal to 525 lines based on the NTSCTV signal in scanning line number conversion circuits 111, 112, 113, then converted into analog signals by D/A converters 118, 119, 120, further limited in bandwidth by low-pass filters 121, 122, 123 and supplied to an NTSC encoder 124.
The luminance signal Y of the HDTV signal, entered from the input terminal 100, is also supplied to a synchronization separation circuit 110 for separating a synchronization signal based on the HDTV signal, which is converted in a synchronization conversion circuit 117 into a synchronization based on the NTSCTV signal and supplied to the above-mentioned NTSC encoder 124.
The NTSC encoder 124 encodes the entered R, G, B and synchronization signals into an NTSCTV signal, which is released from an output terminal 125.
Each of the scanning line number conversion circuits 111, 112, 113 shown in FIG. 1 is so composed, for simplifying the circuitry, as to form the signal of a scanning line of NTSCTV signal by averaging the signal of two scanning lines of HDTV signal, as shown in a block diagram of FIG. 2. The entered HDTV signal is delayed by a horizontal scanning period in a line memory 130 and supplied to an adder 131 which adds the entered HDTV signal and the delayed HDTV signal supplied from the line memory 130. Then a 1/2 multiplying circuit 132 multiplies a coefficient 1/2 to achieve averaging of the HDTV signal corresponding to two adjacent scanning lines on the image frame. This averaged HDTV signal is skipped every other line by a thinning circuit 133 to obtain 525 horizontal scanning lines corresponding to the NTSCTV signal, and is 10 then subjected to the expansion of time based in a time base expansion circuit 134 in such a manner that the horizontal scanning period matches that of the NTSCTV signal. The conversion of scanning lines is thus completed.
However, in the above-explained scanning line conversion of the HDTV signal into the NTSCTV signal in the conventional apparatus shown in FIG. 1, since a horizontal scanning line of the NTSCTV signal is obtained by averaging the signals of two horizontal scanning lines of the HDTV signal, the horizontal scanning lines obtained after such conversion do not constitute a proper interlace structure on the image frame as will be explained in the following, so that the image is distorted in the vertical direction on the display frame.
FIGS. 3A and 3B illustrate the function of the scanning line number conversion circuit shown in FIG. 1. FIG. 3A shows the structure of horizontal scanning lines of the HDTV signal before the conversion of number of scanning lines, seen from a side of the image frame, and FIG. 3B shows the structure of the horizontal scanning lines of the NTSCTV signal after said conversion.
For example, if scanning lines l.sub.0 and l.sub.2 in an (n-1)-th field, shown in FIG. 3A, are averaged to generate a scanning line l.sub.1 ' shown in FIG. 3B, said converted scanning line l.sub.1 ' is positioned, on the image frame, at the center of the scanning lines l.sub.0 and l.sub.2, namely at the position of the scanning line l.sub.1. Similarly a horizontal scanning line obtained from the scanning lines l.sub.1 and l.sub.3 of the n-th field is at the position of the scanning line l.sub.2 on the image frame. Consequently the horizontal scanning lines on the image plane after scanning line conversion are positioned as shown in FIG. 3B, and are equally distanced in each field image, but, in a frame image composed of two fields, for example of (n-1)-th field and n-th field, the horizontal scanning lines are no longer equally distanced, so that the interlaced display in this state results in an image distortion in the vertical direction.